Since a curved surface diffraction grating for a spectrophotometer has capabilities of dispersing and converging light, the number of parts to be used can be reduced, thereby making it possible to simplify the structure of the device. In conventional technologies, a curved surface diffraction grating is fabricated by fabricating a curved surface diffraction grating cast by a method of ruling a curved surface substrate with machine, such as a ruling engine, and by transferring a ruled pattern to resin, metal, or the like.
As a curved surface diffraction grating fabrication method, PCT international publication WO08/081555 discloses a concave blaze-type diffraction grating manufacturing method using photolithography and an ion-beam etching process for semiconductors.
Japanese Unexamined Patent Application, Publication No. Sho 61-72202 discloses a technology in which a diffraction grating pattern is formed on a flexible material, such as resin or a metal thin film, the flexible material is attached to a substrate that is curved so as to have a predetermined curvature, to obtain a cast, the cast is brought into contact with a liquid-state curved surface diffraction grating material, which has not been hardened, and then the curved surface diffraction grating material is hardened, thereby fabricating a curved surface diffraction grating.
Japanese Unexamined Patent Application, Publication No. Hei 8-29610 discloses a technology in which a replica layer (diffraction grating part) using reactive curing resin is laminated on a flexible substrate, and a flat-shaped diffraction grating is curved by utilizing cure shrinkage of the reactive curing resin.
Japanese Unexamined Patent Application, Publication No. Hei 9-5509 discloses a technology in which a flat surface diffraction grating substrate is transferred to a flexible material, and the flexible material is fixed to a curved surface substrate, thereby forming a curved surface diffraction grating cast.
Japanese Unexamined Patent Application, Publication No. 2010-25723 discloses an X-ray reflector manufacturing method that includes a smoothing process of smoothing a surface of a silicon substrate so as to make it available for X-ray reflection; and a plastic deformation process of plastically deforming the silicon substrate by applying pressure and heat with a matrix having a predetermined curved surface shape, thereby making the surface of the silicon substrate have the predetermined curved surface shape.
Among the above-described diffraction grating fabrication methods, in the diffraction grating manufacturing method using semiconductor process, which is described in PCT international publication WO08/081555, it is difficult to accurately form a diffraction grating pattern.
With all the technologies described in Japanese Unexamined Patent Application, Publication No. Sho 61-72202, Japanese Unexamined Patent Application, Publication No. Hei 8-29610, and Japanese Unexamined Patent Application, Publication No. Hei 9-5509, because a flexible material is used at the stage of forming a diffraction grating pattern, pattern accuracy is reduced.
Furthermore, although Japanese Unexamined Patent Application, Publication No. 2010-25723 describes a method of plastically deforming a silicon substrate, this invention relates to an X-ray reflector, and, if this method is used for diffraction grating fabrication, a silicon diffraction grating pattern is also smoothed under a high-temperature and hydrogen atmosphere. Then, plastic deformation needs to be conducted to curve a silicon flat surface diffraction grating, dislocation lines occur, and voids etc. occur when the diffraction grating is fixed to a curved fixing substrate, thus reducing surface accuracy.
The present invention has been made to solve these problems, and an object thereof is to fabricate a curved surface diffraction grating having a desired curvature with high accuracy.